1. Field of the Invention
This invention relates to a plate making method for gravure printing. More particularly, the invention relates to a plate making method for gravure printing in which a continuous tone positive is used as the original pattern for printing and the invention is characterized in that a tint screen or a special contact screen is used in an exposure step, thereby imparting the factors of both laterial sizes and depths to the ink cells of the gravure printing plate so as to attain excellent tone reproducibility in the gravure printing.
2. Description of Prior Art
The gravure printing methods can be roughly divided into two kinds, one is the conventional gravure printing method in which the gradation is represented with ink cells of varied depths, and the other is the inverted halftone gravure printing method using a halftone positive in which the gradation is represented with ink cells of varied areas.
In the conventional gravure printing method, the resist layer having varied thickness is formed by photomechanically curing the gelatin layer of a carbon resist and the variation of the resist thickness is transferred to a plate material such as a copper plate as the variation of depths of ink cells by means of etching proces. Since the carbon resist is used in this method, various unstable factors can not be avoided in the processes of exposing, developing and etching. Especially, the depths of ink cells of highlight portion is generally about 1 micron, however, it is difficult to attain the accurate formation of such depth without fail. While, in the inverted halftone gravure printing method using a halftone positive, sensitizing solution is applied to a plate material and the plate material is exposed to light through a halftone positive, then the exposed plate material is subjected to development and etching. In this method, a large number of fine halftone dots (ink cells) are formed in the range from highlight portion to light portion. In printing opration, the ink transfer from such small ink cells to the suface of printing material is unstable, thus the halftone dots are liable to be dropped out and the appearance of printed surface becomes rough. Especially, when the printing material is paper, the rough feeling grows worse as the surface of paper is also rough.
Further, in order to remedy the shortcomings of the above-mentioned conventional gravure printing method and inverted halftone gravure printing method, it is considered that the gradation is represented by means of both the sizes and depths of ink cells of printing plate. A typical method is called as double positive method, in which two positives are used, that is, a continuous tone positive is used in order to give the factor of cell depth and a halftone positive, to give the factor of cell area. Each positive is separately printed to a photosensitive carbon resist, however, since two kinds of same sized positives are used, several problems are caused to occur in the process work.
For example, in this method, two positives of a continuous tone positive and a halftone positive are necessary for each color, so that when four color printing is carried out, eight positives are required and they should be correctly registered together. Therefore, the reqistering of positives and the retouching become very troublesome and difficult. Further, in the case tht designation of printing is changed after plate making, the use of photosensitive material increases as well as the increase of trouble for retouching or redoing, and the production cost is increased.